Various embodiments of the invention relate generally to a MEMS device and particularly to a method for manufacturing the same.
Membrane structure is one of the crucial elements for acoustic devices, such as microphones. The key parameters of the membrane structure include its compliance, mass, and damping which determine its resonant frequency and quality factor.
Capacitive sensing basically measures the capacitive change due to air gap modulation. The gap is typically defined by a stationary electrode and a moving electrode which modulates the gap. To maximize the signal-to-noise (SNR) of the system, the gap of the capacitor needs to be optimized to make a compromise between maximizing the electrical sensitivity and minimizing the thermo-mechanical (or Brownian) noise due to damping.
To simultaneously achieve high mechanical sensitivity with adequately high resonant frequency and within the limit of supplying/pull-in voltage, the moving mass needs to be minimized. For a given acoustic aperture area which is normally specified by the application, the reduction of mass is equivalent to the reduction of thickness, which leads to the weakening of the mechanical strength of the structure and can often lead to yield issues in production as well as reliability issues during the usage.
One of the problems with conventional two-chip microphones is the requirement for large packaging which adds significantly to the total cost. In addition, parasitic capacitances are undesirably high because the two chips are wire bonded together.
To address the above-noted problems, the CMOS substrate and the MEMS substrate are integrated creating a one-chip solution. One of the significant advantages of this approach is the ability to lower the die cost by integrating CMOS and MEMS substrates through wafer-level bonding. The acoustic sensing membrane is built on the MEMS substrate (or “device”) layer which also acts as the moving electrode and the stationary electrode is built on the CMOS substrate. Packaging size and consequently packaging costs are reduced and savings on footprint or area are realized by the microphone users. However, the electrical gap and acoustic gap become coupled, resulting in a trade-off between maximizing electric sensitivity and minimizing acoustic damping (thermo-mechanical noise) which limits the maximum achievable SNR.
What is desired is a small-packaged, low-cost MEMS device with high signal-to-noise ratio SNR. This is achieved by decoupling the electrical and acoustic gaps without compromising high resonant frequency from greater mass.